Spark plug

Abstract

A spark plug that satisfies the relation 2.8≦(A+B)/M, where A is the sum of a length A1 of contact between a sheet packing and a metallic shell of a spark plug in one half section and a length A2 of contact between the sheet packing and the insulator in the one half section, and B is the sum of a length B1 of contact between the sheet packing and the metallic shell in the other half section and a length B2 of contact between the sheet packing and the insulator in the other half section. M is the difference obtained by subtracting the inner diameter D of a ledge from the inner diameter C of a middle hole portion.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a spark plug.BACKGROUND OF THE INVENTION[0002]One known spark plug includes an insulator that internally holds a center electrode and a metallic shell that internally holds the insulator. In such a spark plug, a sheet packing is held between the insulator and the metallic shell in order to ensure air tightness therebetween (see, for example, WO 2011 / 125306).[0003]When the temperature of the center electrode of the spark plug is excessively high (e.g., 950° C. or higher), pre-ignition occurs in which the center electrode serves as a heat source and causes ignition to occur before spark discharge is generated. In a spark plug, its heat range (heat dissipation properties), which is the degree of dissipation of heat which the center electrode receives as a result of combustion to its surroundings, has been adjusted in order to prevent pre-ignition. One path for heat dissipation from the center electrode is a path extending fro...

AI Technical Summary

Benefits of technology

[0007](1) According to one mode of the present invention, there is provided a spark plug comprising a tubular insulator extending in an axial direction, parallel to an axial line, from a rear end side toward a forward end side, the insulator having a step portion having a surface facing the forward end side; a tubular metallic shell for holding the insulator there inside, the metallic shell including a ledge that supports the step portion and a middle hole portion located on the rear end side of the ledge and connected to the ledge; and a sheet packing held between the step portion and the ledge. In this spark plug, 2.8≦(A+B)/M holds, where A (mm) is a sum of a length A1 (mm) and a length A2 (mm), the length Al being a length of contact between the sheet packing and the metallic shell in one of two half sections obtained by dividing, by the axial line, a cross section of the spark plug that passes through the axial line, the length A2 being a length of contact between the sheet packing and the insulator in the one of the two half sections, B (mm) is a sum of a length B1 (mm) and a length B2 (mm), the length B1 being a length of contact between the sheet packing and the metallic shell in the other one of the two half sections that is different from the one of the two half sections, the length B2 being a length of contact between the sheet packing and the insulator in the other one of the two half sections, and M (mm) is a difference obtained by subtracting an inner diameter D (mm) of the ledge from an inner diameter C (mm) of the middle hole portion. In this mode, the area of contact between the insulator and the sheet packing and the area of contact between the sheet packing and the metallic shell can be ensured sufficiently. Therefore, heat dissipation through a path from the insulator through the sheet packing to the metallic shell can be improved.

[0008](2) In accordance with a second aspect of the present invention, there is provided a spark plug, as described above, wherein an average Vickers hardness E of a portion of the metallic shell that is located at a depth of 0.2 mm from an interface between the metallic shell and the sheet packing in the cross section may be 240 HV or more, and an average Vickers hardness F of the sheet packing in the cross section may be 100 HV or more and less than the average Vickers hardness E. In this mode, the sheet packing is prevented from being deformed excessively to thereby prevent the position of the insulator relative to the metallic shell from being excessively displaced toward the forward end side. In addition, the heat dissipation through the path from the insulator through the sheet packing to the metallic shell can be improved.

[0009](3) In accordance with a third aspect of the present invention, there is provided a spark plug, as described above, wherein the ledge may have an inner surface facing the rear end side, and a thickness of t...

Problems solved by technology

When the temperature of the center electrode of the spark plug is excessively high (e.g., 950° C. or higher), pre-ignition occurs in which the center electrode serves as a heat source and causes ignition to occur before s...

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